1. Field of the Invention
The present invention relates to systems and methods for controlling vibration. More specifically, the present invention relates to systems and methods for suppressing vibrations in missiles.
2. Description of the Related Art
In very dynamic environments, missiles are typically subject to severe vibration and shock during launch egress, flight ascent, and stage separation. If these vibration and shock loads are not mitigated, various system components may be damaged, causing the missile to fail.
Mission success requires that the missile be able to keep the target in its field-of-view while it maneuvers itself into a position to intercept the target. A primary disturbance to the missile is the divert thrust delivered by the propulsion system. This thrust force tends to deform the missile into a beam bending mode at its first natural frequency. If the missile frequency modes (including the seeker frequency mode) have natural periods less than or on the same order as the divert thruster rise time, then significant dynamic amplification and airframe ringing will occur.
The dynamic amplification and the airframe ringing or vibration response make target tracking particularly difficult as the optical elements within the seeker will move relative or out of phase to each other producing significant seeker line-of-sight (LOS) motion. Seeker pixel resolution can be maximized by providing a very rigid missile airframe to minimize the jitter transmitted to the seeker platform.
A missile must also be able to accurately determine its own position in order to compute a flight path to intercept the target. Missiles typically include a guidance system that relies on an inertial measurement unit (IMU) to determine the position of the missile by measuring its acceleration and rotation. The IMU is extremely sensitive and should be very rigidly and precisely mounted to the missile airframe, which should also be very stiff. Otherwise, the IMU will move around and make inaccurate measurements, causing the missile to tumble out of control. The entire forebody assembly should therefore be made as stiff as possible to provide a stable platform for the IMU.
Unfortunately, airframe stiffening for better IMU and seeker performance can lead to undesirable transmission of high frequency vibration and shock loads due to rocket motor ignition, stage separations, aerodynamic buffeting, and acoustic loading. If these vibration loads are coupled to the electronic components, the electronics may be critically damaged, leading to missile failure. In addition, structural stiffening typically results in greater mass and weight, which affects the maneuverability and range of the missile.
Efforts to make the structure more compliant—for example, by using rubber mounts to isolate the electronic components—may attenuate the high frequency vibrations, but excessive structural compliance may disable accurate IMU displacement and rotational readings with respect to the missile trajectory. A significant challenge that is faced when packaging electronics equipment is therefore the trade off between providing sufficient isolation from separation and divert shock loading, versus sufficient stiffness to enable IMU platform functionality, while still meeting strength and weight requirements.
In addition, missile systems must typically be designed to attenuate flexible body dynamics or the system could have self-exciting vibrations. In the case where these vibrations are not bounded, catastrophic structural damage and mission failure may occur. In the case where the vibrations remain finite, the additional frequency content in the actuator commands can lead to actuator failure due to overheating and mission failure. Currently, digital notch filters are used to attenuate the effects of the lower frequency modes (1st, and 2nd lateral modes, 1st torsional, and fin modes) and low-pass filters to attenuate the effects of the higher frequency modes. A problem with this approach is that the use of digital filters results in phase loss at low frequencies, which limits the robust performance of the flight control system. The notches associated with the 1st lateral body mode are usually the lowest frequency modes and have the greatest impact on robust performance of the flight control system.
The traditional approach to these problems is to physically tune the structural responses of the missile components and assemblies (including the electronics housings and mounting structures, as well as the airframe and airframe joints) to mitigate these vibration loads. This process typically involves iterative, long term dynamic analyses of the individual components and assemblies. This highly detailed FEM analysis results in dynamic transfer functions incorporated into system guidance simulation evaluations, where further optimization is usually necessary, resulting in tuning requirements for the airframe again per analysis, iterating the transfer function and simulation studies. Several different designs may be constructed and tested at great expense before a satisfactory design is found. This procedure has proven to be extremely time consuming, wrought with errors, and has led to, significant program development schedule slippages and cost overruns.
Hence, a need exists in the art for an improved system or method for mitigating missile vibration loads that is simpler, less expensive, and less time consuming than prior approaches.